Micromachined gas-filled chambers and method of microfabrication

ABSTRACT

Micromachining, etching and bonding techniques are employed to fabricate hermetically sealed gas-filled chambers from silicon and/or glass wafers. The hermetically sealed gas-filled chambers have precise dimensions and are filled with a preselected concentration of gas, thus rendering exceptional performance for use as an optical gas filter. The first step involves etching one or more cavities or holes in one or more glass or silicon wafers. These wafers eventually become part of a chip assembly having one or more hermetically sealed gas-filled chambers after appropriate bonding procedures. Interfaces between aligned silicon wafers are bonded using fusion bonding techniques whereas interfaces between silicon and glass wafers are bonded using anodic bonding techniques. Bonding is accomplished in an over-pressured gas-filled bonding environment that contains a selected concentration of gas which is maintained at the bonding temperature in order to encapsulate a precise concentration of the gas within the micromachined cavity.

This application is a divisional of application Ser. No. 09/012,660filed on Jan. 23, 1998, now U.S. Pat. No. 6,124,145.

FIELD OF THE INVENTION

The invention relates to the microfabrication of gas-filled chambersmade from silicon and/or glass wafers using anodic and/or fusion bondingtechniques. The method is extremely well-suited for microfabricatingoptical filter components, such as those used in optical transducers toidentify and measure the constituents of anaesthetic and breathing gasesin medical applications.

BACKGROUND OF THE INVENTION

Optical infra-red filters have become key components for many infra-redsensors. For instance, carbon dioxide filters which consist of ahermetically sealed carbon dioxide filled chamber having windows areused in medical respiratory applications. Such filters are typicallyexpensive and their lifetime is normally limited because the sealing ofthe chamber has to be made at chip level using adhesives, or othersuboptimal bonding technique. The invention was developed in an effortto improve the fabrication of optical gas filters and related componentssuch as infra-red radiation sources using wafer level siliconmicromachining techniques which have started to become more practical inrecent years.

Micromachining techniques have made it possible to fabricate differentmicromechanical components having structure details with dimensions ofthe order of micrometers and main dimensions perhaps on the order ofmillimeters. Micromachining techniques are related to methods used inthe manufacturing of semiconductors, for example various structures areformed in silicon crystal directly by etching with the aid of aprotecting mask, or by growing different thin films on the surface ofthe silicon crystal via vaporizing, sputtering, printing or other thinfilm techniques known to those who manufacture integrated circuits. Theassignee of the present application has been involved in the developmentof micromachining techniques to fabricate various components forinfra-red sensors such as the infra-red radiation source assemblydisclosed in U.S. Pat. No. 5,668,376 entitled “Double Radiation SourceAssembly And Transducer” by Weckstrom, et al., issued on Sep. 16, 1997to the assignee of the present application, herein incorporated byreference. The ultimate goal of the invention disclosed in theabove-referenced patent, as well as the invention disclosed in thisapplication, is to use wafer level silicon micromachining techniques toimprove the fabrication process of gas sensors used to measurerespiratory gases present in patients' airways during anesthesia orintensive care. The use of effective micromachining techniques not onlyleads to miniaturized components, but often also leads to more accurateand durable components.

BRIEF SUMMARY OF THE INVENTION

In one aspect, the invention employs the use of micromachining, etchingand bonding techniques to fabricate hermetically sealed gas-filledchambers from silicon and/or glass wafers. The hermetically sealedgas-filled chambers have precise dimensions and are filled with apreselected concentration of gas, thus providing exceptional performancefor use as an optical gas filter. The microfabricated gas-filledchambers are also durable. The use of fusion and anodic wafer bondingtechniques leads to a completely hermetically sealed chamber which isdurable even under conditions in which repeated thermal cycling occurs.

The first step in implementing the invention involves etching one ormore cavities (or holes) in one or more glass or silicon wafers. Thewafers eventually become part of a chip assembly having one or morehermetically sealed gas-filled chambers after appropriate bondingprocedures. Preferably, etching techniques are used to create cavitiesin a silicon substrate comprised of one or more silicon wafers.Interfaces between aligned silicon wafers are bonded using fusionbonding techniques. The silicon substrate with the etched cavity is thenplaced within a gas-filled anodic bonding environment. The gas-filledanodic bonding environment contains a selected concentration of gaswhich is maintained at the anodic bonding temperature T_(ab) andpressure P_(ab) while a glass wafer is aligned on the silicon substratewith the etched cavity. (It may be desirable to replace the glass waferlid with a silicon wafer having a glass coating.) Voltage is applied toanodically bond the glass wafer to the silicon substrate. During thisprocess, gas from the anodic bonding environment is encapsulated insideof the cavity at the same concentration and pressure that the gas ispresent in the anodic bonding environment. The composition of the gaswithin the anodic bonding environment, as well as the concentration andpressure P_(ab) of the gas, is predetermined in order that thecomposition and concentration of the gas encapsulated within the gaschamber are sufficient for the selected application. For instance, whenfabricating a gas-filled chamber which is to be used as an optical gasfilter at ambient temperatures (such as used in respiratory gases sensorsystems) the anodic bonding environment will typically contain carbondioxide gas at an inflated pressure such as two atmospheres (i.e. anover-pressurized carbon dioxide environment) thereby allowing theoptical gas sensor to obtain a reference signal that exactly matches theabsorption spectrum of carbon dioxide. Carbon dioxide gas filters have astrong absorption peak at 4.2 μm. In some embodiments of the invention,the optical path length through the chamber is precisely defined by thethickness of the silicon wafers. In order to obtain maximum absorption,the final anodic bonding is implemented in a bonding environmentcontaining carbon dioxide at a relatively high pressure, therebyallowing more carbon dioxide to be encapsulated in the chamber. Due tothe use of carbon dioxide over-pressure in the bonding environment,higher concentrations of carbon dioxide are possible within the chamber,and the chamber can thus be made physically smaller. Depending on theapplication, the smaller size may significantly simplify fabrication.

Silicon is extremely well-suited for use in carbon dioxide filtersbecause silicon is essentially transparent to radiation having awavelength λ=4.23 μm. However, optical transmission losses can occurthrough the glass and by refraction at the physical interfaces along theoptical path. These optical transmission losses can be reduced bythinning the glass wafers through which the radiation passes, and byusing anti-reflective coatings such as silicon nitride and silicondioxide. Silicon dioxide is the preferred coating because silicondioxide does not affect the quality of fusion or anodic bonds.

As an alternative to anodic bonding a glass wafer lid to the assemblywithin an over-pressured bonding environment, it may be desirable insome circumstances to enclose the chamber while contemporaneouslycapturing a preselected concentration of gas in the chamber using asilicon wafer lid. In fabricating such an assembly, it is necessary tochemically treat the silicon wafers. Initial bonding to capture thepreselected concentration of gas in the chamber can occur at roomtemperature. The gas pressure and concentration in the initial bondingenvironment is substantially the same as the pressure and concentrationof the gas captured in the chamber after fabrication. After initialbonding, the silicon wafers are annealed by fusion bonding to strengthenthe bond and hermetically seal the chamber. Such a microfabricatedstructure has the advantage of simplicity, and also has reduced opticaltransmission losses.

In another aspect of the invention, the above-described techniques areused to microfabricate several gas-filled chambers (e.g. optical gasfilters) on a single chip. The fabrication can be accomplished within asingle gas-filled bonding environment to create several cells having thesame gas composition and concentration. Intentional leaks to thesurrounding atmosphere can be micromachined into the chip so that one ormore chambers communicate with the atmosphere. In this manner, somechambers on the chip will contain a selected gas (e.g. carbon dioxidegas) at a selected concentration while other chambers will remain opento the surrounding atmosphere. Alternatively, the gas-filled environmentbonding technique can be applied in sequence in a variety of gas-filledbonding environments in order that various chambers on the same chip maycontain gases of different concentration and/or composition. In thepreferred method of implementing this aspect of the invention, it isnecessary to saw the wafer used to seal the respective chambers betweensequential bonding processes.

In yet another aspect of the invention, the microfabricated gas-filedchamber or plurality of chambers are integrated with one or moreinfra-red radiation sources on a single chip. The silicon wafer on whichthe one or more IR sources are microfabricated is preferably used toform part of the chamber which contains the gas. In this manner, an IRsource and optical gas filter are efficiently and effectively integratedon a single chip assembly. In many cases, it will be preferred that thesingle chip assembly include a plurality of laterally disposed IRradiation sources microfabricated on a single wafer and bonded to theremainder of the chip assembly to form the respective gas-filledchambers.

Other features and aspects of the invention will become apparent tothose skilled in the art upon reviewing following drawings anddescription thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing illustrating the structure of amicrofabricated, hermetically sealed chamber filled with carbon dioxidegas in accordance with the preferred embodiment of the invention.

FIG. 2 is a schematic drawing illustrating anodic bonding in accordancewith the invention within an over-pressured anodic bonding environmentdesigned to provide a customized concentration of gas with thehermetically sealed chamber.

FIG. 3 is a graph illustrating test results of optical transmissionthrough a carbon dioxide filter made in accordance with the inventionshown as a function of wavelength for various anodic bondingover-pressures.

FIGS. 4a and 4 b are graphs illustrating optical transmission lossesthrough glass wafers.

FIG. 5 is a graph illustrating optical transmission losses through asilicon wafer having silicon oxide coatings of various thickness forwavelength λ=4.23 microns.

FIG. 6 is a graph illustrating optical transmission losses through thegas-filled chamber illustrated in FIG. 1 provided that a thin glasslayer and silicon oxide coatings are used to reduce optical transmissionlosses.

FIG. 7 schematically illustrates various alternative embodiments formicrofabricating a hermetically sealed gas-filled chamber in accordancewith the invention.

FIG. 8 is a schematic drawing illustrating anodic bonding within anover-pressured anodic bonding environment using a silicon substratehaving an etched cavity and a silicon wafer having a glass coating toenable anodic bonding.

FIG. 9 is a schematic drawing illustrating the initial bonding of asilicon substrate having a cavity etched therein and a silicon wafer lidwithin a gas-filled bonding environment, the initial bonding being laterfollowed by fusion bonding at high temperatures to strengthen the bond.

FIG. 10 schematically illustrates various methods of applying ananti-reflective coating to reduce refractive optical transmission lossesin a gas-filled chamber used as an optical filter.

FIG. 11 illustrates a single-chip, microfabricated assembly whichincludes a plurality of chambers (most hermetically sealed andgas-filled) in accordance with the invention.

FIG. 12 is a schematic drawing illustrating one method of fabricatingthe single-chip, microfabricated assembly shown in FIG. 11.

FIG. 13 contains schematic drawings illustrating the preferred manner ofmicrofabricating the single chip microfabricated assembly shown in FIG.11.

FIG. 14 contains schematic drawings showing another embodiment of thesingle chip microfabricated assembly shown in FIG. 11.

FIG. 15A shows a transducer using a gas-filled chamber microfabricatedin accordance with the invention.

FIG. 15B shows the transducer of FIG. 15A connected to a breathingcircuit of a patient.

FIG. 16 is a schematic view illustrating a microfabricated carbondioxide filter chip having a carbon dioxide chamber used in connectionwith an infra-red radiation source chip in which the reference radiationsource is laterally disposed from the sampling radiation source on thesame chip.

FIG. 17 is a schematic drawing illustrating the preferred steps infabricating an infra-red radiation source chip having laterally disposedinfra-red radiation sources on the same chip.

FIG. 18 is a top view of the chip shown in FIG. 16 which includeslaterally disposed infra-red radiation sources.

FIG. 19 contains schematic drawings showing the various embodiments ofthe invention in which hermetically sealed gas-filled chambers areintegrated with a plurality of laterally disposed infra-red radiationsources on a single chip.

FIG. 20 is a schematic drawing containing various embodiments of theinvention in which a plurality of hermetically sealed gas-filledchambers are contained on a single chip and a plurality of laterallydisposed infra-red radiation sources are also integrated on the samechip.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 6 relate to a first embodiment of a microfabricatedcarbon dioxide optical filter 10 a, 10 b made in accordance with theinvention. The filter 10 a, 10 b as shown in FIG. 1 comprises carbondioxide gas 14 encapsulated within a chamber 12. The chamber 12 as shownin FIG. 1 has outer dimensions of approximately 2.8 millimeters×2.8millimeters and a cavity depth of 1 millimeter. The filter 10 a, 10 b ismade by etching and bonding three pure, single crystalline siliconwafers 16 a, 18 a, 20 a, (or 16 b, 18 b, 20 b), and a Pyrex glass wafer22 which are best suited for the bonding techniques disclosed herein.

The microfabrication process for the filter 10 a, 10 b is shownschematically in FIG. 1. Fabrication starts with the oxidation of two500 μm thick pure silicon wafers 18 a and 20 a (or 18 b and 20 b). Anoxide layer 24 is patterned on both sides by double side alignmentphotolithography, see FIG. 1A. This is followed by standard KOHanisotropic etching to form holes 26 a, 26 b through the wafers 18 a, 20a. Note that by slightly prolonging the etching through the wafers, theinclined walls (26 a in FIG. 1B) are eliminated and straight walls 26 b(shown in FIG. 1B′) are formed instead. After the holes 26 a, 26 b areformed in the pure silicon wafers 18 a and 20 a (or 18 b and 20 b), theremaining oxide layer 24 is removed. The two etched silicon wafers 18 aand 20 a, or respectively 18 b, 20 b, are then aligned and joinedtogether using a fusion bonding procedure. The silicon wafer lid 16 a or16 b is then fusion bonded to the top wafer 18 a (or 18 b) having a holeetched therethrough. The result is a triple stack wafer substrate 16 a,18 a, 20 a (or 16 b, 18 b, 20 b) with a one millimeter deep cavity 12 a(or 12 b).

In implementing the fusion bonding technique, the bonding surfaces ofthe silicon wafers are cleaned and pretreated to enable the surfaces tochemically bond with each other. The wafers are then aligned and heatedat approximately 1000° C. in a nitrogen atmosphere for several hours toanneal or bond the silicon wafers.

The glass wafer 22, preferably a Pyrex 7740 glass wafer, is thenanodically bonded to the triple stack silicon substrate 16 a, 18 a, 20 ausing a suitable bonder such as a Karl Suss SB6 Bonder. This bonder hasa variable pressure chamber (vacuum to three bar) with an electrostaticoption that allows the glass wafer 22 to be separated from the siliconsubstrate before bonding. Using the electrostatic option, the glasswafer (in vacuum) is separated, and then the variable pressure chamberwithin the bonder is ventilated with carbon dioxide gas. Referring nowto FIG. 2, when the desired pressure of carbon dioxide gas P_(ab) isreached, the glass wafer 22 is joined together with the siliconsubstrate 15, and an otherwise standard anodic bonding procedure isperformed: the preferred anodic bonding temperature T_(ab) is 430° C.;the preferred anodic bonding voltage is 1500 volts; and the preferredanodic bonding time is approximately 1 hour. Since the anodic bonding isperformed at an elevated temperature T_(ab) (e.g. 430° C.), the internalpressure P_(c) within the gas chamber 12 after cooling (e.g. ambienttemperature T₀) is approximately half of the bonding pressure P_(ab). Inother words, anodic bonding is performed at carbon dioxide over-pressure(i.e. P_(ab)=P_(c)×the ratio T_(ab)/T₀) in order to encapsulate a higherconcentration of carbon dioxide gas within the chamber 12. If desired,the four stack wafer assembly (FIG. 1) is then sawed into individualfilters 10. The yield attained from this process is typically betterthan 95%. The success of fusion and anodic bonding ensures very goodhermetic seals of the carbon dioxide chambers 12. Thermocycling testshave also been conducted on chambers 12 fabricated in accordance withthe above-described process, and the chambers 12 did not reveal any gasleaks.

FIG. 3 illustrates optical filter characteristics of the filter 10 shownin FIG. 1 in which the anodic bonding pressure P_(ab) is varied. In FIG.3, the transmission spectrum of the carbon dioxide chamber 12 is plottedversus wavelength, for three different bonding pressures P_(ab) (1, 1.5and 2 bar). The plot in FIG. 3 confirms the strong absorptioncharacteristics of carbon dioxide at 4.23 μm. As expected, the anodicbonding pressure P_(ab) has a strong effect on the absorption filter. Ata pressure of one bar, the maximum absorption is 49%, while at apressure of 2 bar, the maximum absorption reaches almost 90%.

Silicon is essentially transparent to infra-red radiation at 4.23 μmwhich is wavelength of interest with a carbon dioxide gas filter 10.However, optical transmission losses can occur at λ=4.23 μm as theoptical path passes through the glass wafer 22, and also due toreflection at the physical interfaces between dissimilar materials(especially at interference with silicon). FIG. 4a is a plotillustrating the amount of optical transmission loss through a 500millimeter thick Pyrex 7740 glass wafer versus wavelength λ with air asreference. In FIG. 4a, note that glass absorption is dominant at about 5μm or larger which means that Pyrex glass cannot be used for thesewavelengths. Note also that at a wavelength λ=4.23 μm, a 500 μm thickglass wafer has an optical transmission loss of approximately 60%. Toimprove optical transmission, it is desirable to reduce the thickness ofthe glass wafer 22. FIG. 4b illustrates optical transmission losses atλ=4.23 μm for various thicknesses for the glass wafer 22. Note that 70%transmission can be achieved if the glass wafer 22 is thinned to 100 μm.If necessary, thinning can be achieved via etching.

Due to the large change in the refractive index from air to silicon,there are especially high reflection losses at these interfaces. Tominimize these losses, the surfaces of the silicon along the opticalpath can be coated by anti-reflective coatings. The preferredanti-reflective coating is a silicon dioxide coating because such acoating does not interfere substantially with bonding procedures. FIG. 5shows the percentage of optical transmission losses at λ=4.23 μm as afunction of thickness for a silicon dioxide coating. As shown in FIG. 5,the preferred thickness of silicon dioxide anti-reflective coatings isapproximately 0.75 μm. Another suitable anti-reflective coating issilicon nitride. Silicon nitride can be used effectively as ananti-reflective coating, however, the fabrication procedure is morecomplicated because the silicon nitride coating needs to be patterned sothat fusion bonding is not adversely affected. Thus, with a siliconnitride coating, an extra mask and an extra photolithography step arenecessary as well as a sensitive alignment step.

FIG. 6 compares the performance of an optical gas filter 10 shown inFIG. 1 after the glass wafer 22 is thinned and anti-reflected coatingsare used. Curve 28 shows the optical transmission versus wavelength fora carbon dioxide gas filter 10 having a 500 μm glass wafer 22, noanti-reflective coating, a bonding pressure of 2 bar, and a cavitythickness of 1 millimeter. Curve 30 shows optical transmission versuswavelength for a carbon dioxide filter having a 125 μm glass wafer 22,0.575 micron silicon dioxide anti-reflective layers, a bonding pressureof 2 bar, and a cavity thickness of one millimeter. Note that theperformance of the optimized filter (curve 30) is substantially better,thereby improving signal contrast.

FIG. 7 illustrates various alternative embodiments for microfabricatinggas-filled chambers 12 using micromachining techniques similar to thosedescribed in FIGS. 1-6. In FIG. 7a, the silicon substrate 15 comprisestwo silicon wafers 16, 18. A hole 12 is dry edged through the secondsilicon wafer 18. The silicon wafers 16 and 18 are fusion bonded, andthen the glass wafer 22 is anodically bonded in a gas-filled anodicbonding environment to capture a concentration of gas within the chamber12. The embodiment shown in FIG. 7a is similar to that shown in FIG. 1except the width of the chamber 12 extends for only the length of asingle silicon wafer 18. FIGS. 7b and 7 c are embodiments similar tothat shown in FIG. 7a. In FIG. 7b, the hole 12 etched through the secondsilicon wafer 18 has sloped walls as is typical with a KOH etch. In FIG.7c, the walls of the hole 12 are KOH etched from both sides. FIG. 7fshows an embodiment in which the silicon substrate consists of a singlesilicon wafer into which cavity 12 is etched to form the chamber 12after anodic bonding of the glass wafer 12 to the silicon wafer 15. InFIG. 7g, the cavity 12 is etched into the glass wafer 22 rather than thesilicon wafer 15.

In FIG. 7d, the first silicon substrate 16 is replaced by glass wafer32. Glass wafer 32 is anodically bonded to the silicon wafer 18 havingthe hole 12 etched therethrough. In the embodiment shown in FIG. 7d, itis necessary that only one of the anodic bonds (i.e. either glass wafer22 or glass wafer 32) to be implemented within the gas-filled anodicbonding environment. In FIG. 7e, the hole 12 is etched through a glasswafer 35 rather than a silicon wafer. Also, the glass wafers 22, 32shown in FIG. 7d are replaced with silicon wafers 34, 15.

In FIG. 7h, the glass wafer 22 shown in FIG. 7a is replaced with asilicon wafer 22 a having a glass coating 22 b. In this manner, it ispossible to use anodic bonding to bond the silicon substrate 15 with thecavity 12 to the silicon wafer lid 22 a. The layer 22 b of bondableglass can be deposited on the silicon wafer 22 a using liquid glassspin-on techniques, Pyrex sputtering, or some other glass depositiontechnique. FIG. 8 shows the anodic bonding technique for the embodimentshown in FIG. 7h. Note that anodic bonding occurs in an over-pressuredanodic bonding environment as disclosed in detail with respect to FIG.2, the primary difference being the application of the bondable glasslayer 22 on the silicon wafer 22 a.

In FIG. 7i, the glass wafer 22 shown in FIG. 7a is replaced with anothersilicon wafer 34. The silicon wafer 34 is fusion bonded to silicon wafer18 after an initial bond implemented within a bonding environmentcontaining a gas, such as carbon dioxide gas. The gas (e.g. carbondioxide) is captured within the chamber 12 during the initial bondingstep. Referring to FIG. 9, the fusion bonding technique is now explainedin more detail. First, the silicon substrate 15 and silicon wafer lid 34are chemically prepared in acid. After chemical preparation, the siliconsubstrate 15 and the wafer lid 34 are aligned in a fixture with a smallgap between the bonding surfaces. It is important that the bondingsurfaces do not come in contact with one another prematurely. Thealigned silicon substrate 15 and silicon wafer lid 34 are then placed ina gas-filled environment having a preselected concentration and pressureat room temperature (i.e. the preselected gas pressure will be the samepressure in the chamber 12 after complete fabrication). The siliconwafer is then pressed against the silicon substrate 15 for initialbonding at room temperature. The initially bonded structure is thenheated to strengthen the bond and hermetically seal the gas within thechamber 12. Preferably, heating is accomplished in two steps: first, themicrostructure is heated to approximately 400° C. within the bondingmachine, and later it is heated in a furnace to a temperature higherthan 400° C., such as 1000° C.

FIG. 10 shows various configurations that may be useful for reducingoptical transmission losses including various manners of applyinganti-reflective coatings. In FIG. 10, a hole 12 is etched in the glasswafer 35 and two silicon wafers 16, 34 are anodically bonded to theglass wafer 35 to form the gas-filled chamber 12. Silicon dioxideanti-reflective coatings 36 are applied continuously over the siliconwafers 16, 34. The optical path 38 passes through the wafers 34, 16 andthe respective anti-reflective coatings 36 as well as the gas-filledchamber 12. In FIG. 10, the silicon dioxide coatings 36 are replacedwith silicon nitride coatings 40 which must be patterned and aligned asto not interrupt the bonding interfaces between the respective wafers34, 35 and 16. The embodiment in FIG. 10 is similar to that in FIG. 7aexcept that the glass wafer 22 has been thinned by etching and siliconnitride coatings 40 have been applied to reduce optical transmissionlosses.

FIG. 11 shows schematic drawings illustrating various embodiments of theinvention in which a plurality of chambers 12 are microfabricated onto asingle chip assembly 42. In FIG. 11, each of the chambers 12 is filledwith the same gas at the same concentration. In FIG. 11, each of thechambers 12 is filled with a different gas composition or different gasconcentration. Separate glass covers 44 are provided for each of thevarious gas compositions/concentrations. In FIG. 11, a conduit 46 ismicrofabricated into wafer 18 (although the conduit 46 could bemicrofabricated into any of the wafers 22, 18, 16). The conduit 46 leadsfrom chamber 12L in FIG. 11 to an edge of the chip 42 so that thechamber 12L is open to the outside atmosphere. Thus, the chamber 12Lwill be filled with e.g. ambient air when the chip assembly 42 is inoperation.

FIG. 12 shows one possible method of microfabricating a single-chipassembly having gas-filled chambers 12 a, 12 b filled with differentgases and/or gas concentrations. First, glass wafer 22 is anodicallybonded to silicon wafer 18 to encapsulate gas A within chamber 12 a.Then, glass wafer 32 is anodically bonded to the other side of siliconwafer 18 to capture gas B within chamber 12 b. The single-chip assembly42 shown in FIG. 12 is, however, suboptimal because the double use ofglass wafers 22, 32 is likely to lead to excessive optical losses.

FIG. 13 illustrates the preferred manner of microfabricating an assemblyhaving a plurality of hermetically sealed gas-filled chambers 12 a, 12b, 12 x on a single chip 42. FIG. 13a shows glass wafer 44 a which isanodically bonded to the silicon substrate while located in an anodicbonding environment containing a selected concentration of gas A. Theglass wafer 44 a has been etched so that portions of the glass wafer 44a not covering cavities 12 a selected to contain gas A or the respectivesurrounding bonding surfaces for the selected cavities 12 a do notcontact the silicon substrate 15 when the glass wafer 44 a is alignedwith the silicon substrate 15 for anodic bonding in the environmentfilled with gas A. After anodic bonding, the glass wafer 44 a is sawedto remove the portion of the glass wafer 44 a which was previouslyetched away and was not covering the cavities 12 a selected to containgas A (see FIG. 13b). FIG. 13c shows the use of a second glass wafer 44b which again has portions etched away. The second glass wafer 44 b isanodically bonded to the assembly while being located in an anodicbonding environment containing gas B (see FIG. 13c). After anodicbonding of the second glass wafer 44 b to the silicon substrate 15, gasB is encapsulated within the respective hermetically sealed chambers 12b, and again the excess portions of the glass wafer 44 b are removed bysawing. This procedure can continue for as many gas environments asdesired (see FIG. 13e).

FIG. 14 shows yet another alternative fabrication method formicrofabricating a plurality of hermetically sealed gas-filled chambers12 a, 12 b, 12 x on a single chip 46. In FIG. 14a, a cavity 12 a isetched in silicon substrate 15 a which is fabricated from a singleetched silicon wafer. The pre-etched silicon wafer 15 a is anodicallybonded to the glass wafer 22 within an anodic bonding environmentcontaining gas A, thereby capturing gas A within the chamber 12 a. Afteranodic bonding of silicon wafer 15 a to the glass wafer 22, the excessportion of the silicon wafer 15 a is sawed with a dicing blade to removethe excess portion. This process can be repeated for multiple gasenvironments as shown in FIGS. 14b and 14 c.

A typical application for an optical filter 10 as disclosed thus far isshown in FIGS. 15A and 15B. FIG. 15A shows a transducer 48 and adetachable a gas sample volume 50. If the transducer 48 is used as amainstream transducer, the walls 52 and the windows 54, 56 of the gassample volume 50 are parts of a connector 58 (FIG. 15A) which is part ofthe patient's breathing circuit, but which can be detached from thetransducer 48. The connector 58 (FIG. 15A) should be disposable, orshould be able to be easily sterilized. Therefore, it is an importantcharacteristic that the connector 58 be detachable from the transducer48. The transducer 48 includes an infra-red radiation source chip 68 anda carbon dioxide filter chip 70 containing a carbon dioxide-filledchamber 12. The chips 68/70 can be held by a ceramic holder, or can beintegrally fabricated as disclosed in FIGS. 18 and 19. The IR sourcechip 68 and the gas filter chip 70 are mounted to casing 71 a which inturn is mounted to transducer housing 73. In a similar manner, casing 71b holds the components on the other side of the transducer 48.

The IR source chip 68 includes a plurality of filaments 72 which emitradiation through the carbon dioxide filter 70, through window 74,through sample gas volume 50, through window 76, through IR band passfilter 78 and onto infra-red detector 80. The glass windows 70, 76 canbe of glass, ceramic, silicon or other thin material transparent toinfra-red radiation. If necessary, an anti-reflective film should beused to reduce reflective optical transmission losses. Typically, the IRsource 72 includes an array of 20 to 50 microfilaments. As described inthe above incorporated U.S. Pat. No. 5,668,376, the IR radiation source68 is fabricated from a pure silicon substrate. Briefly, a recess isetched in the substrate in the region of the microfilaments, i.e. in theregion which emits radiation. The filaments are typically made ofpolycrystalline silicon which is doped to become conducting andprotected by a silicon nitride coating, but they may be made by othermaterials suitable for thin film technology such as metal (e.g.tungsten) etc. The filaments are spaced away from the bottom surface ofthe recess, such that the microfilaments have a small thermal capacityand thus a short thermal time constant. The volume of an individualmicrofilament is at most 200,000 μm³, preferably at most about 50,000μm³, and typically of the order 2,000 to 20,000 μm³; and the thermaltime constant of the microfilaments is at most about 50 milliseconds,preferably at most about 10 milliseconds, typically of the order 0.5 to5 milliseconds. Thus, a single microfilament has a typical thickness ofthe order of 0.5 to 5 μm, a width of the order 5 to 100 μm, and a lengthof the order 50 μm to 3 millimeters. The distance of the filament fromthe bottom of the recess is generally of the order 10 to 1,000 μm,typically of the order 50 to 300 μm. The filaments of the IR source 72are electrically connected to metal electrodes on the IR chip which actas terminals to the rest of the electrical system. In FIG. 15A,electricity is supplied to IR chip 72 via wire 82.

FIG. 15B shows a patient's breathing circuit. Transducer 48 andconnector 58 are connected between the patient's incubation tube 84 andthe Y-piece 86. The Y-piece connects the input hoses 88, 90 of device 92which maintains breathing. The transducer 48 is connected electronicallyvia line 94 and connector 96 to the patient monitor 98, where the signalis processed. The patient monitor 98 includes a display 100 which showsthe concentration of the gas being measured by the transducer 48 as afunction of time, i.e. the breathing curve or the concentration valuesof inhalation and exhalation.

The described infra-red analyzer is primarily intended to be used in themainstream as a carbon dioxide transducer, but using the same principalit would also be possible to measure other gases or liquids which absorbradiation. In anesthesia, it is primarily a question of laughing gas oranesthesia gas. It is quite clear that there are also other gases thatcan be measured in other applications.

FIG. 16 illustrates a particularly useful configuration for an IR sensorwhich has been developed as a result of the ability to fabricate a chip104 having a plurality of hermetically sealed gas-filled chambers 12.FIG. 15 illustrates a microfabricated carbon dioxide filter chip 104used in connection with an infra-red radiation source chip 102 in whicha reference radiation source 106 is laterally displaced from thesampling radiation source 108 on the same chip 102. FIG. 18 shows a topview of the IR source chip 102. Note that there are two diagonallydisposed reference sources 106 a, 106 b and two diagonally disposedsampling sources 108 a, 108 b. The carbon dioxide filter chip 104 shouldinclude a separate carbon dioxide-filled chamber 12 for each referencesource 106 a, 106 b. Aluminum bonding pads 110, 112, 114, 116 and 118are provided on the IR source chip 102 and serve as electrodes for therespective radiation sources 106 a, 106 b, 108 a, 108 b. Referring nowto both FIGS. 17 and 18, the IR sources 106 a, 106 b, 108 a, 108 bconsist of silicon nitride 120 encapsulated phosphorous dopedpolysilicon filaments suspended across a respective KOH etched cavity.The filament length is approximately 1 millimeter which is a compromisebetween mechanical strength and thermal characteristics of the IRsource. Shorter filaments would mean a higher mechanical strength, butwould provide a less even temperature distribution over the filamentarea. In this application, the IR sources 106 a, 106 b, 108 a, 108 b areswitched diagonally in pairs between the sampling beam and thereferencing beam in relatively fast modulation (t<ten milliseconds),therefore preferred cavity depth is between 100 and 300 μm.

The fabrication process is shown schematically in FIG. 17. A puresilicon substrate 102 with a 1000 angstrom thick sacrificial polysiliconlayer in the areas of the etched cavities underneath the filaments isprovided to begin fabrication. The sacrificial polysilicon isphotolithographically patterned and dry etched to define the cavityareas in which the filaments will be located. This is followed by a 1 μmstress-free silicon nitride layer and then a 0.5 μm polysilicondeposition. Phosphorous ion implantation, photolithography, andpolysilicon dry etching are implemented to form the doped polysiliconfilament which is then followed by a second 0.5 μm stress-free siliconnitride deposition. Activation of the dopants in the polysiliconfilaments for one hour at 1000° C. gives the sheet resistance of about25 ohms per square. Photolithography and a dry nitride etch open up thecontact holes and a thin (1000 angstroms) stress-free silicon nitride isdeposited to protect the contact holes during the KOH etch. The openingsbetween the filaments, down to the sacrificial polysilicon layer, arethen photolithographically patterned and dry etched. KOH etching resultsin lateral etching of the sacrificial polysilicon and vertical etchingof the silicon substrate which forms cavities having a depth ofpreferably about 200 μm beneath the filaments. Finally, the top siliconnitride layer is removed by a dry etch, and this is followed bydeposition of 1.5 μm thick aluminum layer patterned to form the metalbonding pads 110, 112, 114, 116 and 118. Dicing by sawing results in theIR source chip 102 shown in FIG. 18. The IR source chip shown in FIG. 18can be implemented in an operational transducer as a separate componentfrom the carbon dioxide filter chip 104 as shown in FIG. 16, or it canform a part of an integral single-chip assembly integrating both theinfra-red radiation sources and the respective carbon dioxide filters.

FIGS. 19 and 20 illustrate various microfabricated assemblyconfigurations integrating IR sources and optical gas filters on asingle chip. The IR sources 106, 108 are fabricated on the first siliconwafer 16 and the gas-filled chamber 12 is etched in the second siliconwafer 18. Preferably, as described earlier with respect to FIG. 7, thesilicon wafers 16, 18 are fusion bonded to form the integral siliconsubstrate 15 having the cavity 12 and also the IR sources 106, 108 (IRsources as prefabricated on silicon wafer 16). The glass wafer 22 isthen anodically bonded to the silicon substrate 15 within a gas-filledanodic bonding environment to capture gas within the chamber 12. FIG.19a shows an embodiment in which the reference signal from reference IRsource 106 passes through gas-filled chamber 12, whereas the samplingsignal from the sampling source 108 passes through the silicon substrate15. In FIG. 19b, the sampling signal from the sampling source 108 passesthrough chamber 12 b which contains a leak 46, thereby causing thechamber 12 b to be filled with ambient air. The embodiment shown in FIG.19c is similar to the embodiment shown in FIG. 19c except the chipassembly includes a protective glass cover 124 to protect the IRfilaments 106, 108 from mechanical damage. In FIG. 19d, the IR filaments106, 108 are vacuum-sealed within the glass cover 124, thereby furtherprotecting the filaments 106, 108 from other types of environmentalirritants. Note that the protective cover 124 in FIG. 19d is preferablyapplied in a separate anodic bonding process than the glass wafer 22.Also note that the IR filaments 106, 108 are located external to thegas-filled chamber 12 in FIGS. 19a-19 d. By contrast, FIG. 19e shows anembodiment in which the IR filaments 106, 108 are located within thechambers 12 a, 12 b. Chamber 12 a is gas-filled, such as with carbondioxide gas, so that the reference signal from the reference source 106passes through the gas in the chamber 12 a. On the other hand, chamber12 b includes a leak 46. Thus, the sample signal from the sampling IRsource 108 passes through ambient air rather than a filtering gas.

FIG. 20 illustrates two embodiments in which a single-chip assemblyincluding a plurality of IR sources can be integrated on a single chiphaving a plurality of hermetically sealed chambers 12 a, 12 b, 12 xwherein each chamber 12 a, 12 b, 12 x contains a different gascomposition and/or concentration. In many ways, the method offabrication is similar to that disclosed with respect to FIG. 13. InFIG. 20a, the first silicon wafer 16 is prefabricated to contain IRsources 150 a, 150 b and 150 x before fusion bonding to the secondsilicon substrate 18 and the subsequent sequential anodic bondings ingas A environment for glass cover 44 a, in gas B environment for glasscover 44 b, and in gas X environment for glass cover 44 x, etc. As isevident from the drawing in FIG. 20a, the IR source filaments 150 a, 150b and 150 x are external to the respective gas-filled chamber 12 a, 12b, 12 x in the single-chip assembly shown in FIG. 20a. In FIG. 20b, theIR source filaments 150 a, 150 b and 150 x are located within therespective gas-filled chambers 12 a, 12 b, 12 x. In the single-chipassembly shown in FIG. 20b, the second silicon wafer 16 is prefabricatedto include IR filaments 150 a, 150 b, 150 x as well as the respectivehole 12 a, 12 b, 12 x for the respective gas-filled chamber. The firstand second silicon wafer 16, 18 are then fusion bonded before sequentialanodic bonding of glass cover 44 a in a gas A environment, glass cover44 b in a gas B environment, and glass cover 44 x in a gas Xenvironment.

The invention has been described with respect to various preferredembodiments of implementing the invention. It is apparent that theinvention may be implemented in modified form to microfabricateessentially equivalent structures. The following claims should beinterpreted to include such modifications and equivalents.

We claim:
 1. A single-chip, microfabricated assembly for presentingpreselected optical transmission properties to electromagneticradiation, said assembly comprising: a substrate having a plurality ofcavities etched therein such that a respective anodic bonding surfacesurrounds each cavity, said substrate being transmissive toelectromagnetic radiation; at least one cover bonded to the respectiveanodic bonding surfaces on the substrate to form a plurality ofhermetically sealed gas-filled chambers in the assembly, said at leastone cover being transmissive to electromagnetic radiation; wherein eachrespective cover is bonded to the respective anodic bonding surface onthe substrate by anodic bonding an enclosing wafer to the respectivebonding surfaces on the substrate while the substrate with the pluralityof etched cavities is located in a gas-filled anodic bonding environmentcontaining a selected concentration of gas at an anodic bondingtemperature T_(ab) and pressure P_(ab), the gas being selected to haveoptical transmission characteristics suitable for providing thepreselected optical transmission properties for electromagneticradiation, each respective anodic bond being implemented by aligning theenclosing wafer against the anodic bonding surfaces for the respectivecavities in the substrate to form a bonding interface between theenclosing wafer and the respective bonding surface on the substrate,contemporaneously capturing the selected concentration of gas within therespective cavity to establish, from the optical transmissioncharacteristics of the gas, the preselected optical transmissionproperties for electromagnetic radiation passing through the respectivecavity, and thereafter applying an anodic bonding voltage across therespective bonding interface while the enclosing wafer and the substrateare located in the gas-filed environment at the anodic bondingtemperature T_(ab) and pressure P_(ab) to bond the respective interfaceand hermetically seal the respective cavities with the selectedconcentration of gas therein.
 2. A single-chip, microfabricated assemblyas recited in claim 1 wherein the enclosing wafer is a glass wafer andthe substrate having a plurality of cavities etched therein is a siliconsubstrate having a plurality of cavities etched therein.
 3. Asingle-chip, microfabricated assembly as recited in claim 1 in which thecover is a glass wafer and the substrate is a silicon substrate, and inwhich the anodic bonding of the respective glass wafers is accomplishedin the following manner: the silicon substrate having the plurality ofcavities etched therein is placed in a first gas-filled anodic bondingenvironment containing a gas in a first selected concentration at theanodic bonding temperature T_(ab) and pressure P_(ab), the gas of thefirst environment having optical transmission characteristics suitablefor providing preselected optical transmission properties forelectromagnetic radiation; a first glass wafer is etched such thatportions of the glass wafer not covering a cavity or cavities selectedto contain the gas in the first selected concentration of the firstanodic bonding environment or the respective surrounding bondingsurfaces on the silicon substrate do not contact the silicon substratewhen the glass wafer is aligned with the silicon substrate for anodicbonding; the first glass wafer is anodically bonded to the siliconsubstrate when the silicon substrate is located in the first anodicbonding environment containing the gas in the first selectedconcentration; the first glass wafer is sawed after anodic bonding toremove the portion of the glass wafer not covering the selected cavityor cavities; and the silicon substrate is placed in a second gas-filledanodic bonding environment having the same or different gas in the sameor different concentration at the anodic bonding temperature T_(ab) andpressure P_(ab), the gas of the second environment having opticaltransmission characteristics suitable for providing preselected opticaltransmission properties for electromagnetic radiation; and a secondglass wafer is anodically bonded to the silicon substrate while thesilicon substrate is placed in the second gas-filled anodic bondingenvironment.
 4. A single-chip, microfabricated assembly as recited inclaim 3 wherein the anodic bonding of the respective glass wafers isfurther accomplished in the following manner: the second glass wafer isetched so that portions of the second glass wafer not covering a cavityor cavities selected to contain the gas in the second anodic bondingenvironment or the respective surrounding bonding surface on the siliconsubstrate do not contact the silicon substrate when the second glasswafer is aligned with the silicon substrate for anodic bonding; and thesecond glass wafer is sawed after anodic bonding so that the portion ofthe second glass wafer not covering the cavity or cavities selected tocontain the gas in the second anodic bonding environment is removed fromthe chip.
 5. A single-chip, microfabricated assembly as recited in claim1 wherein the substrate having a plurality of cavities etched thereincomprises a single glass wafer having a plurality of cavities etchedtherein, and the enclosing wafer is a silicon wafer.
 6. A single-chip,microfabricated assembly as recited in claim 1 in which at least one ofthe hermetically sealed gas-filled chambers contains a first gas at afirst preselected concentration, and another of the hermetically sealedgas-filled chambers contains the same gas at a different preselectedconcentration.
 7. A single-chip, microfabricated assembly as recited inclaim 1 wherein at least one of the hermetically sealed gas-filledchambers contains a first gas at a selected concentration and at leastone of the hermetically sealed gas filled chambers contains a differentgas at the same or different preselected concentration.
 8. Asingle-chip, microfabricated assembly as recited in claim 1 wherein thesubstrate having a plurality of cavities etched therein comprises asilicon wafer having a hole etched therethrough which is anodicallybonded to a glass wafer.
 9. A single-chip, microfabricated assembly asrecited in claim 8 in which the enclosing wafer is a glass wafer.
 10. Asingle-chip, microfabricated assembly as recited in claim 1 furthercomprising: at least one chamber which is not completely hermeticallysealed; and an open channel extending from the chamber which is notcompletely hermetically sealed to an edge of the chip to provide anintentional leak from the surrounding atmosphere into the chamber whichis not completely hermetically sealed.
 11. A single-chip,microfabricated assembly as recited in claim 1 wherein the enclosingwafer is a silicon wafer having a bondable glass layer applied thereon.12. A single-chip, microfabricated assembly as recited in claim 1wherein the plurality of hermetically sealed gas-filled chambers arelaterally displaced from each other on the chip.
 13. A single-chip,microfabricated assembly as recited in claim 1 wherein the substrate isa single silicon wafer having a plurality of cavities etched therein.14. A single-chip, microfabricated assembly as recited in claim 1further defined as presenting preselected optical transmissionproperties to infra-red radiation and further comprising an infra-redradiation source which is microfabricated on the same chip as theplurality of hermetically sealed gas-filled chambers.
 15. A single-chip,microfabricated assembly as recited in claim 14 further comprising atleast two infra-red radiation sources which are microfabricated on thesame chip as the plurality of hermetically sealed gas-filled chambers,the infra-red radiation sources being displaced laterally from eachother.
 16. A single-chip, microfabricated assembly as recited in claim14 wherein the infra-red radiation source includes a plurality offilaments which are located within one of the hermetically sealedgas-filled chambers.
 17. A single-chip, microfabricated assembly asrecited in claim 14 wherein the infra-red radiation source includes aplurality of filaments which are located externally of the hermeticallysealed gas-filled chambers.
 18. A single-chip, microfabricated assemblyas recited in claim 14 wherein the infra-red radiation source is alignedoptically with a hermetically sealed, gas-filled chamber.
 19. Asingle-chip, microfabricated assembly as recited in claim 15 whereinsome but not all of the infra-red radiation sources are alignedoptically with a hermetically sealed gas-filled chamber.
 20. Asingle-chip, microfabricated assembly as recited in claim 17 furthercomprising a protective cover which covers the filaments for theinfra-red radiation source.
 21. A single-chip, microfabricated assemblyas recited in claim 20 wherein the filaments are vacuum-sealed withinthe protective cover.
 22. A single-chip, microfabricated assembly forpresenting preselected optical transmission properties toelectromagnetic radiation, said assembly comprising: a silicon substratehaving a plurality of cavities etched therein such that a respectivebonding surface surrounds each cavity, said substrate being transmissiveto electromagnetic radiation; at least one silicon cover bonded to therespective bonding surfaces on the silicon substrate to form a pluralityof hermetically sealed gas-filled chambers in the assembly, said atleast one cover being transmissive to electromagnetic radiation; whereineach respective silicon cover is bonded to the respective bondingsurface on the silicon substrate in the following manner: the siliconsurfaces are prepared for fusion bonding; the silicon substrate andsilicon cover are placed within a gas-filled bonding environment, thegas-filled bonding environment containing a selected concentration ofgas at a selected pressure, the gas being selected to have opticaltransmission characteristics suitable for providing the preselectedoptical transmission properties for electromagnetic radiation; thesilicon substrate and silicon cover are initially bonded together whilethe silicon substrate and cover are located in the gas-filled bondingenvironment by aligning and engaging the respective bonding surfaces onthe respective silicon substrate and cover to form a bonding interfacetherebetween while contemporaneously capturing the selectedconcentration of the gas from the gas-filled bonding environment withinthe respective cavities to establish, from the optical characteristicsof the gas, the preselected optical transmission properties forelectromagnetic radiation; and a sufficient amount of thermal energy isthereafter applied for a sufficient amount of time to strengthen theinitial bonds and hermetically seal the respective chambers with theselected concentration of gas therein.
 23. A single-chip,microfabricated assembly as recited in claim 22 in which bonding of therespective silicon covers is accomplished in the following manner: thesilicon substrate having a plurality of cavities etched therein isplaced in a first gas-filled bonding environment containing a gas in afirst selected concentration, the gas of the first environment havingoptical transmission characteristics; the silicon cover is etched sothat portions of the silicon cover not covering cavities selected tocontain the gas in the first selected concentration of the first bondingenvironment or the respective surrounding bonding surfaces on thesilicon substrate do not contact the silicon substrate when the siliconcover wafer is aligned with the silicon substrate for initial bonding;the initial bonding and fusion bonding procedure is implemented tocapture the gas of the first bonding environment in the first selectedconcentration within the preselected cavities; the first silicon coveris sawed to remove the portion of the silicon cover not covering theselected cavities; the silicon substrate is placed in a secondgas-filled bonding environment having the same or different gas in thesame or different concentration, the gas of the second environmenthaving optical transmission characteristics; and the initial and fusionbonding steps are again implemented with a silicon substrate and anothersilicon cover while the silicon substrate is placed in the secondgas-filled bonding environment, the other silicon substrate being etchedto not cover cavities not selected to contain the gas of the secondbonding environment or the previously bonded covers.
 24. A single-chip,microfabricated assembly as recited in claim 22 in which at least one ofthe hermetically sealed gas-filled chambers contains a first gas at afirst preselected concentration, and another of the hermetically sealedgas-filled chambers contains the same gas at a different preselectedconcentration.
 25. A single-chip, microfabricated assembly as recited inclaim 22 wherein at least one of the hermetically sealed gas-filledchambers contains a first gas at a selected concentration and at leastone of the hermetically sealed gas filled chambers contains a differentgas at the same or different preselected concentration.
 26. Asingle-chip, microfabricated assembly as recited in claim 22 furthercomprising: at least one chamber which is not completely hermeticallysealed; and an open channel extending from the chamber which is notcompletely hermetically sealed to an edge of the chip to provide anintentional leak from the surrounding atmosphere into the chamber whichis not completely hermetically sealed.
 27. A single-chip,microfabricated assembly as recited claim 22 wherein the plurality ofhermetically sealed gas-filled chambers are laterally displaced fromeach other on the chip.
 28. A single-chip, microfabricated assembly asrecited in claim 22 further defined as presenting preselected opticaltransmission properties to infra-red radiation and further comprising aninfra-red radiation source which is microfabricated on the same chip asthe plurality of hermetically sealed gas-filled chambers.
 29. Asingle-chip, microfabricated assembly as recited in claim 28 furthercomprising at least two infra-red radiation sources which aremicrofabricated on the same chip as the plurality of hermetically sealedgas-filled chambers, the infra-red radiation sources being displacedlaterally from each other.
 30. A single-chip, microfabricated assemblyas recited in claim 28 wherein the infra-red radiation source includes aplurality of filaments which are located within one of the hermeticallysealed gas-filled chambers.
 31. A single-chip, microfabricated assemblyas recited in claim 28 wherein the infra-red radiation source includes aplurality of filaments which are located externally of the hermeticallysealed gas-filled chambers.
 32. A single-chip, microfabricated assemblyas recited in claim 28 wherein the infra-red radiation source is alignedoptically with a hermetically sealed, gas-filled chamber.
 33. Asingle-chip, microfabricated assembly as recited in claim 29 whereinsome but not all of the infra-red radiation sources are alignedoptically with a hermetically sealed gas-filled chamber.
 34. Asingle-chip, microfabricated assembly as recited in claim 31 furthercomprising a protective cover which covers the filaments for theinfra-red radiation source.
 35. A single-chip, microfabricated assemblyas recited in claim 34 wherein the filaments are vacuum-sealed withinthe protective cover.